The heart is a pump that pumps blood throughout the body. Cardiac pacemakers have long been used to provide stimulation pulses to the heart in order to control the rate at which the heart pumps or beats, thereby controlling the flow rate at which blood is circulated throughout the body. The principal purpose for circulating blood throughout the body, of course, is to deliver oxygen and other nutrients to the body cells, without which oxygen the body cells would soon die. As the body cells are called upon to do more and more work, the flow rate at which oxygenated blood is delivered to the cells must be increased. This increase in flow rate can be achieved by increasing the rate at which the heart beats or pumps. In a normal, healthy, non-paced heart, the heart rate automatically increases in response to the need to deliver more oxygenated blood to the body cells. However, a pacemaker-controlled heart is unable to automatically increase its rate unless the pacemaker is able to sense that an increased oxygen need is present.
Modern pacemakers include complex stimulation pulse generators as well as cardiac event sensors that can pace or sense in the atrium, the ventricle, or both the atrium and ventricle of the heart. Further, such pacemakers include telemetry capabilities so that the activity of the heart and pacemaker can be transmitted to an attending physician or cardiologist. Advantageously, such pacemakers are also programmable so that the same telemetry capabilities can be used by the attending physician or cardiologist in order to adjust the control parameters associated with operation of the pacemaker. Such parameters not only influence the rate at which the pacemaker's stimulation pulses are generated, but also control the pacemaker's basic mode of operation, i.e., the heart chamber that is paced, as well as the heart chamber that is sensed. Hence, modern pacemakers offer great versitility in the manner of their use. Disadvantageously, many modern pacemakers do not yet have the capability to automatically adjust the pacing rate, or pacing interval, in the absence of a sinus P-wave (a sinus P-wave is explained below) as a function of the body's physiological needs unless some sort of physiological sensor external to the pacemaker is employed. As used herein, the term "physiological need" includes the need to change the flow rate at which oxygenated blood is delivered to the body's cells, as well as other body needs that influence the heart rate.
The present invention is directed to an improved pacemaker that includes the capability of automatically adjusting the paced heart rate as a function of sensed physiological needs within the body. Advantageously, no electronic sensors external to the pacemaker need be employed beyond the normal stimulation leads that are connected between the pacemaker and the heart. As is explained more fully below, the present invention senses physiological need by noting changes in a selected time interval associated with the rhythm of the heart.
In the above-referenced earlier-filed application, of which this application is a continuation-in-part, there was disclosed a system and method for determining P-wave capture. Included in the disclosure of the earlier application, much of which is repeated herein, is a reliable method or system for sensing a P-wave that results from an atrial stimulation pulse. As known to those skilled in the art, a P-wave is generated by the atrium of the heart as it depolarizes. Shortly after depolarization, the atrium contracts, which contraction causes the pumping function of the atrium to be realized. While those skilled in the art will recognize that depolarization and contraction are separate events that do not necessarily occur at the same time, the term "contraction" when used hereinafter means depolarization or an event that always occurs in synchrony with depolarization. Because it is extremely helpful for a physician, cardiologist, or other diagnostician to known when the atrium depolarizes and contracts, and whether this depolarization is a result of a pacemaker stimulation pulse or the result of a natural (non-paced) rhythm associated with the heart, reliably sensing a P-wave using signals sensed through the pacemaker leads, especially a P-wave that occurs in response to a stimulation pulse from a pacemaker, has heretofore presented a formidable challenge. Moreover, because the occurance of a P-wave--the occurance of which represents the depolarization of the atrium--is a key cardiac event that helps define a time interval, the measurement of which is associated with at least one embodiment of the present invention, the system and method described in the foregoing earlier-filed application, or equivalents thereof, comprise an important teaching for practicing the invention herein described. Further, because the system and method of P-wave capture described in the earlier application is not limited to sensing P-waves, but can also be used to sense R-waves resulting from a pacing pulse applied to the ventricle, and because the occurrance of an R-wave is likewise a key cardiac event the measurement of which is associated with at least one other embodiment of the invention, the teachings of the earlier-filed application become doubly important.